The present invention relates to a vacuum arc plasma gun deposition system that can be used to coat relatively large substrates and that can be operated with satisfactory stability for extended periods of time.
Vacuum arc deposition is used to deposit thin films and coatings from a source electrode (usually the cathode) placed in a vacuum chamber and subjected to a high current electrical arc. In the most utilized mode, the electrical current naturally concentrates at minute areas on the cathode surface known as cathode spots, which are heated to very high temperatures. There is very intense local evaporation of the cathode material from the cathode spots. High current densities pass through the vapor emitted from the cathode spot, heating and ionizing the vapor, and thus the emitted vapor expands away from the cathode spot in the form of hypersonic plasma jets. In addition, the vacuum arc produces a spray of molten droplets or solid debris, known collectively as macroparticles. The macroparticles are generally undesirable.
In the 1870xe2x80x2s A. Wright (xe2x80x9cOn the production of transparent metallic films by the electrical discharge in exhausted tubesxe2x80x9d, Am. J. Sci. Arts vol. 13 pp. 49-55 (1877); xe2x80x9cOn a new process for the electrical deposition of metals, and for constructing metal-covered glass speculaxe2x80x9d, Am. J. Sci. Arts vol. 14 pp. 169-178 (1878)) described the application of what was apparently a pulsed vacuum arc to deposit coatings on glass, and described their visual properties. Thomas Alva Edison (xe2x80x9cArt of plating one material with anotherxe2x80x9d, U.S. Pat. No. 526,147, 1894; xe2x80x9cProcess of duplicating phonogramsxe2x80x9d, U.S. Pat. No. 484, 582, 1892) taught the use of a continuous vacuum arc to produce metal coatings, and their use in the process of duplicating phonograms.
Currently, vacuum arc deposition is widely practiced, in particular to deposit diamond-like carbon, TiN, TiCN, (Ti,Al)N, ZrN and other ceramic materials on cutting and forming tools, household hardware (e.g. door knobs, plumbing fixtures), surgical instruments and implants, and jewelry. In the most common xe2x80x9cbatch coaterxe2x80x9d type of configuration, one or more cathodes are mounted in a vacuum chamber and serve as vapor plasma sources. The chamber is periodically opened to remove coated workpieces, and to mount new workpieces for coatings. At these times it is convenient to replace expended cathodes with new ones, and to clean the chamber walls and other components of accumulated coatings and debris. Typical cycle times are on the order of a few hours, during which the arc is operated for only some fraction of the time. In these systems, the coatings will generally contain some degree of macroparticle inclusions.
As taught by Aksenov et al. (Sov. J. Plasma Phys. Vol. 4 p. 425; Pribory I Tekhnika Eksperimenta N5 (1978) p. 1416), macroparticles can be separated from the plasma jets by bending the plasma using a magnetic field around an obstacle that occludes any direct path between the cathode and the substrates. The most common form of obstacle is the walls of a curved duct. Alternatively, as described by S. Falabella and D. M. Sanders, J. Vac. Sci. Technol. A vol. 10 p. 394 (1992), the duct may be formed from straight tubular sections joined at an angle. Nevertheless, some macroparticles may rebound from the duct wall and eventually bounce along the duct and reach the substrate. Several inventions (J. Storer et al., J. Appl. Phys. vol. 66 p. 5245 (1989); R. P. Welty, U.S. Pat. No. 5,480,527) teach that macroparticle transmission may be reduced by corrugating the duct wall or by placing baffle plates in the duct to catch bouncing macroparticles.
Prior art vacuum arc deposition devices are well suited for laboratory studies and for batch coating operation, where there are ample opportunities to replace expended cathodes and to clean the system of accumulated debris. However, in certain applications, long-term stable operation is required. For example, in large flat glass coating plants, an alternative technology, magnetron sputtering, is widely employed, and continuous operation runs of two weeks are common. Stable operation over long periods requires maintaining an approximately constant cathode temperature, electrode geometry and duct geometry, in the face of cathode erosion on the one hand, and the accumulation of a coating on the anode and other surfaces on the other hand.
There is thus a widely recognized need for, and it would be highly advantageous to have, a vacuum arc plasma gun deposition system including mechanisms for stabilizing cathode temperature, electrode geometry and duct geometry.
It is an objective of the present invention to provide the means for stable, long duration, continuous vacuum arc deposition, by providing mechanisms for operating the cathode surface at a constant average temperature, and for maintaining approximately constant electrode and duct geometries in the face of cathode erosion and coating accumulation on other surfaces.
According to the present invention there is provided a vacuum arc plasma gun including: (a) a cathode having an active surface; (b) at least one anode; (c) a current source for causing electrical current to flow from the at least one anode to the active surface of the cathode; and (d) a mechanism for moving the cathode to keep the active surface substantially at a fixed position relative to the at least one anode while the electrical current flows.
According to the present invention there is provided a vacuum arc plasma gun including: (a) a cathode having an active surface and at least one lateral surface; (b) at least one anode; (c) a current source for causing electrical current to flow from the at least one anode to the active surface of the cathode; and (d) a mechanism for cooling the cathode while the electrical current flows, by conducting heat away from the at least one lateral surface.
According to the present invention there is provided a vacuum arc plasma gun including: (a) a cathode; (b) a plurality of anode assemblies defining a channel having a cross sectional size; (c) a current source for causing electrical current to flow from the plurality of anode assemblies to the cathode, thereby causing material to flow away from the cathode via the channel, at least a portion of the material then being deposited on the anode assemblies; and (d) for each anode assembly: a mechanism for moving the each anode assembly to keep the cross sectional size of the channel substantially constant while the material is deposited on the each anode assembly.
According to the present invention there is provided a method of coating a substrate, including the steps of: (a) providing a vacuum arc plasma gun including: (i) a cathode having an active surface, and (ii) at least one anode; (b) causing an electrical current to flow from the at least one anode to the active surface of the cathode, thereby creating a plasma that carries coating material away from the active surface of the cathode; and (c) while the electrical current flows: (i) positioning the substrate relative to the plasma so that at least a portion of the coating material is deposited on the substrate, and (ii) moving the cathode so that the active surface remains substantially in a fixed position relative to the at least one anode.
According to the present invention there is provided a method of coating a substrate, including: (a) providing a vacuum arc plasma gun including: (i) a cathode having an active surface and a lateral surface, and (ii) at least one anode; (b) causing an electrical current to flow from the at least one anode to the active surface of the cathode, thereby creating a plasma that carries coating material away from the active surface of the cathode; and (c) while the electrical current flows: (i) positioning the substrate relative to the plasma so that at least a portion of the coating material is deposited on the substrate, and (ii) removing heat from the cathode by conduction via the lateral surface.
According to the present invention there is provided a method of coating a substrate including: (a) providing a vacuum arc plasma gun including: (i) a cathode, and (ii) a plurality of anode assemblies defining a channel having a cross sectional size; (b) causing an electrical current to flow from the anode assemblies to the cathode, thereby creating a plasma that carries coating material away from the cathode via the channel, a first portion of the coating material being deposited on the anode assemblies; and (c) while the electrical current flows: (i) positioning the substrate relative to the plasma so that a second portion of the coating material is deposited on the substrate, and (ii) moving the anode assemblies to keep the cross sectional size of the channel substantially constant while the first portion of the coating material is deposited on the anode assemblies.
According to the present invention there is provided a vacuum arc plasma gun deposition system for coating a substrate, including: (a) a cathode; (b) at least one anode; (c) a current source for causing electrical current to flow from the at least one anode to the cathode, thereby forming a plasma that flows in a generally axial direction; and (d) a processing section including: (i) a mechanism for moving the substrate substantially perpendicular to the generally axial direction, and (ii) a mechanism for steering the plasma to flow at an angle to the generally axial direction within the processing section.
According to the present invention there is provided a method of coating a substrate, including: (a) providing a vacuum arc plasma gun including: (i) a cathode, and (ii) at least one anode; (b) causing an electrical current to flow from the at least one anode to the cathode, thereby creating a plasma that carries coating material away from the cathode; and (c) while the electrical current flows: (i) moving the substrate past the plasma so that at least a portion of the coating material is deposited on the substrate, and (ii) varying a rate of the flow of the electrical current to vary a rate at which the at least portion of the coating material is deposited on the substrate.
According to the present invention there is provided a method of coating a substrate, including: (a) providing a vacuum arc plasma gun including: (i) a cathode, (ii) at least one anode, and (iii) a processing section; (b) causing an electrical current to flow from the at least one anode to the cathode, thereby creating a plasma that flows into the processing section in a generally axial direction; and (c) while the electrical current flows: (i) moving the substrate within the processing section in a direction substantially perpendicular to the generally axial direction; and (ii) steering the plasma to impinge on the substrate at an angle to the generally axial direction.
The present invention includes four improvements over the prior art.
The first improvement is the provision of a mechanism for moving the cathode so as to keep the active surface of the cathode (i.e., the surface from which the plasma is emitted) at a fixed position relative to the anodes. The cathode is slowly moved axially towards the anodes as cathode material is emitted from the active surface. To stabilize the active surface, the active surface is provided with rounded or chamfered edges.
The second improvement is the provision of a mechanism for cooling the cathode by conducting heat away from the lateral surfaces of the cathode. To enable the cathode to be moved as necessary, this mechanism preferably includes one or more cooling bars that are reversibly urged against respective lateral sides of the cathode. Each cooling bar includes a heat sink and a coolant pipe through which a liquid coolant is circulated to cool the heat sink. An electrically insulating layer, preferably made of either a ceramic or an elastomer, is provided on the side of the heat sink that contacts the cathode. Another electrically insulating layer is provided on the side of the heat sink that faces the anodes.
The third improvement is the provision of anode assemblies that include, in addition to the anodes, respective mechanisms for moving the anodes primarily in an outward direction so that as cathode material is deposited on the anodes, the cross sectional size of the channel defined by the anode assemblies remains substantially constant. Preferably, each anode assembly includes a disposable plate, reversibly mounted on the anode and facing the channel, on which the cathode material accumulates. Most preferably, the disposable plates are made of the same material as the cathode. Preferably, each anode assembly also includes a coolant pipe through which a liquid coolant is circulated to cool the anode.
The fourth improvement is the provision of mechanisms for ensuring that a substrate that is not flat is coated uniformly as the substrate is moved at a constant rate past the plasma. One such mechanism includes one or more coils that steer the plasma to impinge locally perpendicularly on the substrate. The other mechanism involves suitable variation of the current that flows from the anode to the cathode.
The scope of the present invention also includes corresponding methods of using the vacuum arc plasma gun and the vacuum arc plasma gun deposition system of the present invention to coat a substrate, and coated products made by coating substrates using these methods. Among these products are coated architectural glass panes and covers for solar energy collectors.
A vacuum arc plasma gun deposition system of the present invention can operate continuously for at least 24 hours, and often as long as two weeks. This is in contrast to prior art systems, which generally can operate continuously for only up to a few hours.